A semiconductor module is generally manufactured by bonding a ceramic substrate to a base metal, bonding a metal wiring to the ceramic substrate, and bonding an element such as a semiconductor chip to the metal wiring. An adhesive used for bonding is referred to herein as a die bonding material. Solder bonding has been mainly used so far for the bonding of the elements in power devices. However, in view of the requirement for disuse of lead, conductive pastes formed by mixing Ag powder and resin and, lead-free solders have been used as substitute materials.
In the bonding of semiconductor chips, since semiconductor elements such as semiconductor chips have tended to be integrated and refined more, the amount of heat generated per unit area has tended to be accordingly increased. To deal with this problem, semiconductor modules having semiconductor elements mounted thereon need to efficiently dissipate heat generated from the semiconductor elements to the outside. There is a subject of improving the heat conductivity of die bonding materials.
In the bonding of a ceramic substrate, a ceramic substrate mainly comprising AlN or Si3O4 is first metallized. Then, the metallized layer and a semiconductor chip, or the metallized layer and a base metal are bonded to each other by means of a lead-free solder. It is required for a method of forming a strong metallized layer, moderation of difference of heat expansion coefficient between the substrate and the semiconductor chip, improvement in the heat dissipation of the substrate, and long time reliability of such as heat cycle resistance.
Patent Document 1 describes a lead-free solder paste. There is provided a lead-free solder paste containing a Bi solder powder and a Cu metal powder and having a heat conductivity of 52 W/mK.